1. Field of the Invention
The field of this invention relates piezoelectric ceramic transducers and embedding such piezoelectric transducers in composites. More specifically, this relates to a method for embedding piezoelectric ceramic transducers in thermoplastic composites which are much tougher and as strong and stiff as thermoset composite materials, but have a higher consolidation temperature.
2. Related Art
Piezoelectric ceramic transducers have found many important applications in adaptive structures for vibration control and acoustic noise suppression in modern space, civilian and military systems, such as launch vehicles, space platforms, aircraft, submarines and helicopters. These transducers may be attached to or imbedded in advanced composite materials to form adaptive structures that are light weight, strong, and affordable both in manufacturing costs and long operating life. There have been several programs supported by the government to develop technology for the manufacturing and processing of advanced adaptive structures built with advanced composite materials incorporating active devices such as sensors and actuators. The following are a few examples. These include those devices described in "Advanced Composites with Embedded Sensors and Actuators (ACESA)" by Boeing Aerospace and Electronics Division, Seattle, Wash., pursuant to Air Force Astronautic Laboratory Contract at Edwards Air Force Base, Calif., Contract F04611-88-C-0053; and a similar program by TRW Space & Technology Group in Redondo Beach, Calif., with the Astronautics Laboratory, Contract F04611-88-C-0054; and the "Synthesis and Processing of Intelligent, Cost Effective Structures (SPICES) Program" sponsored by the Advanced Research Projects Agency (ARPA), to develop cost effective, adaptive material processing and synthesis technologies for smart adaptive structures. In the first two of these programs, the transducer materials utilized were piezoelectric ceramics of lead-zirconate-titanate (PZT) and electrostrictive ceramics of lead-molybdenum-niobate (PMN). The transducers were bonded to or embedded in the host material. The host materials employed were low temperature graphite/epoxy thermoset composites, such as T300/934 (processing temperature in the range of 280.degree. to 350.degree. F.). Such graphite/epoxy materials have the following disadvantages: their toughness is low; they will outgas, and absorb moisture; and they are vulnerable to degradation due to atomic oxygen, ultra violet and other radiation as well as hostile threats. Also of interest is U.S. Pat. No. 5,305,507 to DVORSKY et. al. The DVORSKY Patent described the method of encapsulating piezoelectric actuators for easy embedding thereof or lamination into the composite structure.
In the SPICES program, two materials were used as the host composite materials. The first was a thermoset, fiberglass/epoxy and the second was a thermoplastic, AS4/PEEK composite. The fiberglass/epoxy composites, which are electrically nonconductive, have the same low toughness as the conductive graphite/epoxy used by others. The embedding of ceramic transducers was done by a resin transfer molding (RTM) method and the highest temperature the ceramic transducers were subjected to was during cure at about 250 degrees Fahrenheit. The second composite material, (AS4/PEEK) , has a much higher toughness than the thermoset materials. It outgasses very little. The material has a potential for low-cost fabrication and for simplified joining. The processing temperature for PEEK using the traditional lay-up and autoclave consolidation method for thermoplastic materials was about 600 to 800 degrees Fahrenheit. In the SPICES program, the operation of embedding active transducers and the consolidation of the host ply material was done by a fiber placement process. In this process, the thermoplastic ply is heated by a laser, fed under compaction onto previous plies and completely consolidated to them in one step. The ceramic transducer was sandwiched between a titanium metal frame and loaded manually in a predesigned cutout in the host material. The transducer was then covered with a thin copper shield to protect it from laser exposure every time the laser hit the composite ply tape over it during the tape laying and consolidating steps. Ceramic transducers so processed have a very loose electro-mechanical coupling with the host AS4/PEEK composite, likely due to the method used to embed the ceramic transducers. Results from piezoelectric shunt-damping tests indicated that the transducer's coupling coefficient was very low. When transducers were used as actuators for passive damping control, the measured resonant amplitude reduction was only a few decibels. When they were used as actuators for active vibration control, the signal was so low that a high gain amplifier was required to increase it. Low control authority of transducers was attributed to poor electromechanical coupling between the transducer and the host composite when ceramic transducers were embedded in the AS4/PEEK thermoplastic composite by the fiber placement process.